Nippon Paint Co., Ltd. of Osaka, Japan recently filed Japanese Patent Publications (unexamined) Nos. 107784/1980 and 152183/1980 on phosphating methods of treating iron-based metal surfaces which are particularly suitable for treating manufactured products having complicated surfaces, such as automobile bodies. The above phosphating methods are in use commercially in the automotive industry for pretreating automobile bodies prior to cationic electrocoating, which are the coating processes now used extensively in this industry. The phosphating method of Japanese Patent Publication No. 107784/1980 is carried out by first subjecting the metal surface to a dipping treatment with an acidic aqueous solution containing 0.5 to 1.5 g/l of zinc ion, 5 to 30 g/l of phosphate ion, and 0.01 to 0.2 g/l of nitrite ion and/or 0.05 to 2 g/l of m-nitrobenzene-sulfonate ion at a bath temperature of 40.degree. to 70.degree. C. for 15 seconds or more, followed by spraying with the above solution for 2 seconds or more. The other method, i.e. the method of Japanese Patent Publication No. 152183/80, comprises spraying onto the metal surface an acidic aqueous solution containing 0.4 to 1.0 g/l of zinc ion, 5 to 40 g/l of phosphate ion, 0.01 to 0.2 g/l of nitrite ion and 2.0 to 5.0 g/l of chlorate ion at 40.degree. to 70.degree. C. for 40 seconds or more.
Recently, in the automotive industry, consistent with the aim of further improving corrosion resistance after the application of a siccative coating, steel components which are plated on one surface only with zinc or a zinc alloy have come to be used as materials for automobile bodies. When the processes of the above Japanese Patent Publications are applied to such materials (i.e. to metal components having both iron-based metal surfaces and zinc-based metal surfaces), the iron-based surfaces are provided with a phophate coating film having a low film thickness with uniform and dense cubic or plate-like crystals, as well as excellent adhesion and corrosion-resistance. Such phosphate coating on the iron-based surface is suitable as a substrate for cationic electrocoating. However, in the case of the phosphate coating film formed on the zinc-based surfaces, the resistance to salt water spraying after the application of a cationic electrocoating thereto is insufficient, and secondary adhesion (by immersion test of the film with cross-hatched scratches in warm water) after cationic electrocoating - intermediate coating - top coating is greatly inferior to that on the iron-based surfaces. In addition to the above Japanese Patent Publications, the following references disclose phosphating compositions for metal surfaces:
U.S. Pat. No. 3,338,755 issued Aug. 29, 1967 to Jenkins et al. This patent discloses a process for phosphating metal surfaces with a phosphating solution containing zinc, manganese, phosphate, nitrate, and nitrite, as essential ingredients, in stated proportions.
U.K. No. 983,924 issued Feb. 24, 1965 to Pyrene Co., Ltd. is by the same inventors as U.S. Pat. No. 3,338,755 and has the same general disclosure.
German No. 29 31 693, issued Dec. 11, 1980 to Fosfa-Col, discloses a phosphating process using a solution containing zinc, manganese, phosphate, nitrate, and chlorate ions in stated gram-atom relationships. However, none of the above proposed phosphating methods have succeeded in giving satisfactory results with the above combination of substrate materials.
Japanese Patent No. J50139-039 (JA No. 197511) discloses a conversion coating solution containing manganese ions for the treatment of zinc surfaces. However, this prior art solution contains from 3 to 20 g/l of zinc ions, which will result in a conversion coating having leaf-like crystals on iron-based surfaces. Such leaf-like crystals are unsuitable as a substrate for cationic electrocoating. Hence, the solutions of this patent are unsuitable for treating both zinc-based and iron-based surfaces.